Flood control structures in tidal creeks, sloughs harm fish in lower Fraser River

Hundreds of small flood-control structures in the lower Fraser River are robbing salmon and other fish of valuable habitat while creating conditions for non-native species to flourish, according to a new research study.

The study looked at five sites with flood-control structures and five without in the Allouette, Pitt and Fraser systems, 44 to 57 kilometres from the ocean.

Juvenile salmon were 2.5 times more abundant at the sites without floodgates and 11.7 times greater specifically for coho salmon, while prickly sculpin and native minnow were 37.2 and 11.7 times more abundant, respectively.

Non-native fish species such as pumpkinseed, brown bullhead and common carp were 3.1 times more abundant at floodgate sites, according to the study published Thursday in the Canadian Journal of Fisheries and Aquatic Sciences, published by NRC Research Press.

Lead author David Scott conducted the research for his master’s thesis at Simon Fraser University and is now a researcher with Raincoast Conservation Foundation.

The Vancouver Sun paddled Katzie Slough in Pitt Meadows for two kilometres with Scott, ending at floodgates and a pump station at Kennedy Landing next to the Pitt River. Salmon fry could be observed outside the station, unable to migrate upstream into the slough to feed and rest in relative safety.

“That’s good habitat up there,” Scott confirmed. “But they won’t be able to get there.”

Ian Hinkle of Watershed Watch Salmon Society called on all levels of government to work together to resolve the issue, noting old infrastructure “didn’t take into consideration the salmon.”

The destructive power of larger dams to fish habitat is well known, but the influence of smaller-scale flood-control structures is much less documented. Floodgates typically are designed to close during high tides and the spring freshet to prevent upstream flooding into farm fields and urban areas.

The problem is that the floodgates can also block the passage of fish and create low-oxygen zones in which certain non-native species can survive. Pump houses often associated with the floodgates can also be lethal to migrating fish. In late summer and fall there may also not be enough downstream flow to open the floodgates, a problem that could be resolved by forcing the gates open.

Researchers note that coastal flood risks are expected to increase with climate change and rising sea levels leading to improved flood-control structures and an opportunity to implement designs friendlier to native fish.

A total of 30,759 fish of 21 species were caught in seine nets at the 10 sites in the study.

The study estimates there are about 500 small flood-control structures in the lower Fraser River.

The five study sites with floodgates: McLean Creek and Fenton Slough, which feed into the Pitt River; Cranberry Slough, into the Alouette River; and Yorkson Creek and Nathan slough, into the Fraser River. The five sites without structures: De Boville Slough and Smokwha Marsh, into the Pitt River; McKenny Creek into the Alouette; and West and Nathan creeks, into the Fraser.

Each of the 10 sites was sampled once per month from April to August during one year.

In August, dissolved oxygen levels in all floodgate sites fellow below Ministry of Environment minimum standards for protection of aquatic life. Low oxygen extended for at least 100 metres upstream of floodgates.